新疆巴什苏洪稀有矿床碱性杂岩体地球化学特征及地质意义

Geochemical characteristics and geological significance of alkaline complex in Bashishuhong rare deposit,Xinjiang

新疆巴什苏洪稀有稀土矿床处于西南天山南缘和塔里木盆地北缘,构造位置优越。针对巴什苏洪碱性杂岩体进行了系统的调查和采样,对各个岩性进行了岩石学和地球化学研究。结果表明,辉长岩具有富Fe_(2)O_(3)^(T)、MgO、CaO、TiO_(2)、P_(2)O_(5),贫SiO_(2),呈碱性、准铝质等特点,高场强元素(HFSE)相对于大离子亲石元素(LILE)的富集程度较低,具有明显的Ba正异常,不具有明显的Nb-Ta负异常。霓石正长岩、碱长花岗岩具有富SiO_(2),贫Fe_(2)O_(3)^(T)、TiO_(2)、P_(2)O_(5)等,大多呈碱性的特点,有从过铝质到过碱质演化的特征,明显亏损Ba、Sr、P、Ti,富集Rb和K,稀土元素蛛网图表现为平缓、轻微右倾的海鸥式分布模式,Eu的负异常十分明显。岩石学和岩石地球化学特征表明辉长岩是由软流圈地幔部分熔融,经过一定的分离结晶作用,上升侵位形成的产物;碱长花岗岩由软流圈地幔的底侵导致地壳重熔,经过结晶分异作用,上升侵位形成;霓石正长岩由结晶分异后的基性岩浆和酸性岩浆发生混合作用,再经过一定程度的分离结晶作用,最后上升侵位形成。辉长岩和霓石正长岩与后期的陆内裂谷体系有关,碱长花岗岩经历了陆内裂谷的构造环境。

Bashisuhong rare and REE deposit in Xinjiang is located in the southern margin of Southwest Tianshan Mountains and the northern margin of Tarim Basin.The tectonic position is favorable.In this paper,detailed investigation and sample collection is carried out for Bashisuhong alkaline complex,and the petrology and geochemistry of each rock type is studied.The result shows that the gabbro is rich in Fe_(2)O_(3)^(T),MgO,Cao,TiO_(2),P_(2)O_(5),poor in SiO_(2) with an alkaline and quasi aluminous feature.The enrichment degree of HFSE is lower than that of LILE,with the obvious positive anomaly of Ba and no obvious negative anomaly of Nb and Ta.Aegirine syenite and alkali feldspar granite are rich in SiO_(2) and poor in Fe_(2)O_(3)^(T),TiO_(2),P_(2)O_(5),etc.,and most of them are alkaline and have the characteristics of evolution from the peraluminous to the peralkaline.They are obviously depleted in Ba,Sr,P,Ti and rich in Rb and K.The chondrite-normalized REE diagram shows a gentle and slightly right dipping seagull distribution pattern,and the negative anomaly of Eu is very obvious.According to the petrological and geochemical characteristics,it is shown that the gabbro is the production of partial melting of asthenosphere mantle and ascending emplacement after certain fractional crystallization.The alkali feldspar granite is the production of crustal remelting caused by underplating of asthenosphere mantle and ascending emplacement after crystallization differentiation.The aegirine syenite is the production of mixture of basic magma and acid magma after crystallization differentiation.After a certain degree of fractional crystallization,it finally forms for the ascending and emplacement.Gabbro and aegirine syenite are related to the later intracontinental rift system.The alkali feldspar granite has experienced the tectonic environment of intracontinental rift.